Inverted responses of the carbon cycle to orbital forcing in Mesozoic peri‐platform marginal basins: implications for astrochronology

Astrochronology depends on the faithful record of insolation forcing in climatic proxies, including the carbon isotope composition measured on bulk carbonates (δ13Ccarb). In marginal basins close to carbonate platforms, the source of carbonate is varied, which can impact the record of the astronomical cycles in the δ13Ccarb signal. We compare here the δ13Ccarb values together with detrital and weathering proxies before and during a crisis in the platform carbonate production (Weissert event, Valanginian, ~135 Ma) to document how a change in the carbonate source can affect the record of the orbital forcing by the carbon isotope system. The level of burial diagenesis was insufficient to alter the clay mineral assemblages which are linked to cyclic changes in weathering conditions. The δ13Ccarb values correspond to the values measured in other parts of the basin which experienced various levels of burial diagenesis, suggesting that they also reflect a paleoenvironmental signal. In marl beds, the δ13Ccarb values increase with detrital and kaolinite content suggesting that humid/arid cycles controlled the evolution of the δ13Ccarb signal in marl beds. Before the Weissert event, the δ13Ccarb values in the limestone beds increase with CaCO3 content and arid conditions. This can reflect the change of type of carbonate produced in shallow‐marine environments and exported to the basin. These environmental changes disrupted the record of the eccentricity cycles in the δ13Ccarb signal. The sources of carbonate must therefore be clearly identified and documented before using the δ13Ccarb series for orbital tuning in hemipelagic areas close to carbonate platforms.